Digitally controlled overload relay

ABSTRACT

An overload relay is provided which monitors current flow in a circuit and triggers an electromagnetic interrupter to open the circuit upon detection of an overcurrent condition. A train of pulses is generated and received by the electromagnetic interrupter for maintaining the circuit closed. The pulse train is terminated upon detection of an overcurrent condition.

RELATED APPLICATIONS

This application is a division of application Ser. No. 365,861 filedApr. 5, 1982, now U.S. Pat. No. 4,456,871, entitled "Power Supply forElectronic Control System".

The subject matter described in this application is related to materialdisclosed in co-filed patent applications, now issued U.S. Pat. No.4,446,498 "Control System for Overload Relay or the Like"--F. Stich;U.S. Pat. No. 4,423,458 "Signal Processing System for Overload Relay orthe Like"--F. Stich; and U.S. Pat. No. 4,423,459 "Solid State OverloadRelay Control and Method"--F. Stich and C. Williams.

BACKGROUND OF THE INVENTION

The present invention relates to a simple and inexpensive power supply,and more particularly to a power supply of the type which automaticallyvaries the firing angle of a thyristor in order to control and outputvoltage. A further aspect of the invention is the provision of a"watchdog" circuit which operates a circuit interrupter if a train ofoutput pulses is terminated or disrupted.

Circuit protection devices such as circuit breakers, relays, contactorsand the like are commonly used for disconnecting electrical circuitsupon the detection of undesired currents. In addition to breaking thecircuits in which the currents flow, other functions may be providedsuch as actuating alarms and safety devices, or the control of otherapparatus in response to a sensed current characteristic. While inprinciple the opening of an electrical circuit in response toundesirably high currents is a simple procedure, in practice theoperation of such protective devices is highly complex owing to thevarious, often conflicting requirements of electrical systems.

For instance, while it is necessary to protect electric motors from highcurrents which could damage or destroy the windings, in order to start amotor under load a high initial current is required. Also, during theoperation of various electrical equipment, for instance under changingloads, high current flow must be tolerated for short periods of time.Further a single "threshold" for current flow cannot be assigned since asmall overcurrent condition can be tolerated far longer than a highovercurrent condition. For these reasons industrial relays andcontactors are commonly provided with complex control mechanisms whichmake use of two or more different current-responsive stages in anattempt to "tailor" the tripping characteristics of the device to adesired application.

In principle it is known that more sophisticated control systems can bedesigned to replace the present electromechanical, magnetic, and thermalcontrols. Most such controls must derive their operating voltage frompower lines which carry relatively high voltages. For commercial reasonsthe devices must be usable with a broad range of voltages, often from100 to 500 volts. The voltage of the control power supply, however, mustremain within a closely-regulated band of the order of 10 volts. At thesame time the power supply must be simple, rugged and relativelyinexpensive.

Recently efforts have been made to design electronic control systemswhich make use of digital and other allied signal processing techniqueswhich will provide the desired functional flexibility, and eliminate theneed for mechanical adaptations to change the range or operatingcharacteristics of a control. Two examples of such a system are shown inU.S. Pat No. 4,219,858--DePuy et al and U.S. Pat. No. 4,219,860--DePuy.These patents disclose an overcurrent relay control which utilizesdigital sampling, multiplexing and signal accumulation techniques fordetecting overcurrent conditions in one or more phases of a multiphaseelectrical system.

The nature of the direction exercised over the output device, such as arelay winding, operated by the control system is also a critical factorand, particularly with sophisticated and complex control systems, it isdesirable to provide a form of "fail-safe" output control in order to becertain that outputted signals cannot be misread and that improperfunctioning or anamolous signals in the control system will not resultin catastrophic failure. It will therefore be appreciated that it wouldbe highly desirable to provide an improved circuit interrupting controlsystem which supervises interrupter operation in a "fail-safe" manner,and to provide a voltage supply stage which is relatively inexpensivebut is capable of accepting applied voltages whose values vary over awide range.

It is therefore an object of the present invention to provide animproved control operating apparatus for circuit interrupters of theoverload relay and contactor type.

Another object is to provide an interrupter control which monitors thestatus of the interrupter and responds in a "fail safe" mode to controlfailure.

Another object is to provide an inexpensive solid-state power supplywhich utilizes elements to convert a broad range of available voltagesinto a regulated, low-voltage supply.

It is a further object of the invention to provide a low costphase-angle controlled power supply which automatically converts a widerange of available line voltages into a single, low-level voltagesuitable for use with digital control equipment.

SUMMARY OF THE INVENTION

Briefly stated, in accordance with one aspect of the invention theforegoing objects are achieved by providing a power supply incorporatinga thyristor in series with a source of high potential, and a gatingcircuit comprising a Zener diode across the power supply feeding an R-Ccircuit coupled to the thyristor gate. A capacitor coupled to thethyristor cathod provides a source of output current, and also an outputand reference voltage. As the input voltage alternates, each half-wavecauses the gate capacitor to charge in a ramp-like fashion. Thethyristor is gated on when the gating capacitor voltage exceeds thevoltage upon the output capacitor. As output voltage rises theconduction angle of the thyristor becomes lower and lower andconversely, so that the firing angle of the thyristor is caused to varyautomatically as an inverse function of output voltage. This maintainsthe output voltage at a relatively stable level. A digital controlsystem for an overcurrent relay which is energized by the power supplyoutputs a train of pulses to a keep-alive circuit which maintains theenergization of an output relay winding so that the relay contactsmaintain a desired position as long as the control system isoperational. Failure of the control system, or anomalous operation whichresults in an irregular pulse train output, results in the disabling ofthe circuit interrupter.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention will be better understoodfrom the following description of a preferred embodiment taken inconjunction with the accompanying drawings in which:

FIG. 1 is a functional schematic diagram illustrating the operation of asystem utilizing the present invention;

FIG. 2 illustrates a load current waveform;

FIG. 3 represents waveforms at various points in the system of FIG. 1;

FIG. 4 illustrates a composite current waveform in an unbalancedmultiphase circuit;

FIG. 5 is a schematic diagram of a presently preferred embodiment of theinvention; and

FIG. 6 illustrates voltage waveforms at various points within the powersupply circuit.

DESCRIPTION OF A PREFERRED EMBODIMENT

In the embodiment of the invention which is shown in the functionalschematic diagram of FIG. 1, an electric circuit including conductors 10is protected by means of an interrupting mechanism, here shown as acontactor comprising contacts 11 and an actuating coil 12. Current isapplied to the actuating coil by an output relay including contacts 13and coil 14. Coil 14 is in turn energized by a driver stage 15, hereinfunctionally depicted as AND gate. Alternating load current I_(c)flowing through the conductors is sensed by current transformers CT1,CT2 and CT3. The signals outputted by the current transformers areprocessed by a suitable circuit, herein shown as rectifier stage 16, toprovide a single rectified AC current signal which is supplied to ananalog-to-digital converter 17 which includes a comparator 18. As willbe more fully described hereinafter, rectifier stage 16 is connectedbetween a point of reference potential V_(r) and an input terminal ofcomparator 18. The polarity of the rectifier stage opposes the referencevoltage, and the maximum voltage output of the rectifier stage issubstantially the same as the reference voltage so that an inverserelationship is established between the rectifier output and the voltagesignal applied to the comparator. The other input of comparator 18 isconstituted by a ramp-like waveform 20 which may generally described asa monotonically changing voltage value and which is produced by a rampgenerator 21. Signals at a sampling frequency f_(s) from a pulsegenerator CLK periodically reset the ramp generator to begin a newsampling cycle. The pulse train also passes through AND gate 15 formaintaining coil 14 in an energizing state.

The change in state of the output of comparator 18, shown herein as asquare waveform 22, is applied to a digitizer 23 which may take the formof a 4-bit counter. The counter is caused to output a second, digitalsignal comprised of pulses on lines 24, the digital signal resolving thevalue of the level of current in conductors 10 into 16 discrete steps.The digital signal is applied to the inputs of a conversion register 25.The register, which in a preferred embodiment is a read only memory(ROM), serves to transform the four-bit current signal into third,current level signals in response to the applied digital signal. Thethird signal, denominated I_(i), has a digital value which is anonlinear function of the second signals received from counter 23 andray comprise an 8-bit digital word. The digital current signal I_(i),which in the present nomenclature represents the ith sample of current,is applied to one of two signal paths by means represented as switch 26.A counter 27 outputs control pulses which coincide with, but are at alower frequency than, the sampling frequency f_(s). In this manner,depending upon the setting of counter 27 by its "Trip Class" input,every nth current sample signal is directed to the lower signal path;the balance of n-1 sample signals are applied to the upper path, asshown. I_(i) is transmitted to an arithmetic processing stage 28 such asa counter which is supplied with a value corresponding to a desiredcurrent threshold I_(t). The current signal I_(i) is added algebraicallyto the threshold value I_(t) and the difference, which may be positiveor negative, is loaded into an accumulator 29 which may be a counter,shift register or similar device.

A digital representation of the current sample is also applied to adecoder 32, which in turn drives an annunciator stage, here shown asLED1. While the digital signal may be derived at various points of thesystem in a presently preferred embodiment the four-bit signal fromcounter 23 is used.

Accumulator 29 continually sums the current signals from unit 28.Positive-valued signals, indicating an overcurrent, increase the valueof the digital signal stored within accumulator 29 while negative-valuedsignals, representing current below the threshold value, are subtractedfrom the accumulated sum. The content of accumulator 29, correspondingto an accumulated digital count, is fed back through a dividing stage 30where it is divided by a factor Q, and thence to a summing node 31 andused to establish a current threshold value I_(t). In this manner thenet current threshold level will reflect the thermal history of the loaddevice as manifested by current flow and the content of the accumulatorwill stabilize at an appropriate level in the presence of normal currentflow.

When the sensed current level is too great the accumulated value exceedsthe accumulator trip threshold, as established at comparator 38, and theaccumulator in effect overflows and causes a TRIP signal to be produced.The TRIP signal is applied to AND gate 15 for disabling it andpreventing pulses from CLK to be transmittal for energizing coil 14.Contacts 13 then open, deenergizing coil 12 and opening contacts 11. Atthe same time accumulator 29 is decremented in a manner to be explainedhereinafter.

The current signals which are not diverted to the lower, or overcurrent,signal path are applied by switch 26 an arithmetic unit 35 and tested todetermine whether they satisfy the inequality P I_(i-1) ≦I_(i)≦(I/P)I_(i-1) in order to detect samples which deviate from eachprevious sample I_(i-1) by more than a given percentage. Signals whosevalues satisfy both sides of the inequality increment counter M, whilethose signals which deviate from the preceding ones by more than thedesired bandwidth increment counter J. Counter M and each section J₁ andJ₂ of counter J produces an output when it is filled, counter M having asubstantially larger content than the J counter. If counter M is filledfirst, it outputs a signal which resets both itself and all sections ofcounter J. On the other hand, if a section of counter J is filled beforecounter M, representing a predetermined incidence of fluctuation incurrent level, a signal is outputted which changes either or both thecurrent threshold values I_(t) and trip threshold T_(t).

It will now be understood that for a relatively high incidence ofout-of-tolerance current signals, counter section J₁ will be filledbefore counter M can overflow and output a reset signal. The output ofcounter stage J₁ is added to the fed-back "thermal memory" signal fromaccumulator 29, causing current threshold I_(t) to be lowered andincreasing the net value (I_(i-) I_(t)) which is applied to accumulator29. This has the effect of causing the current-related signals to beaccumulated at a substantially constant rate despite an imbalancebetween phase currents, or other distortion of the circuit waveforms.

In like manner should the population of out-of-tolerance current signalsin a given sample be still greater, indicating a still more unbalancedor even single-phased situation, counter segments J₁ and J₂ will both befilled before main counter M outputs its reset signal and accordingly, astill larger value will be applied to summing node 31 for decreasingthreshold signal I_(t) still further. This provides further compensationand maintains the rate at which current signals are accumulated for agiven level of circuit current flow, and also maintains the point atwhich the accumulator content will stabilize in the absence of anovercurrent condition. In this manner the system is caused to maintainits sensitivity to overcurrent conditions despite varying degrees ofcurrent imbalance.

Although the present invention may be used with single-phase systems, itis anticipated that it will most commonly be used with three-phasecurrent. As is well known by those skilled in the art, half-waverectified three-phase current exhibits a regular ripple configuration,as shown in FIG. 2. The duration of each of the regular waveforms issubstantially 120° of the full cycle of each phase waveform. Although itis easily within the skill of the art to filter the rectified currentand reduce the ripple therein in order to provide a practically constantcurrent level which is representative of current flow in the circuit,the present inventor has found that this approach is actuallyundesirable when used with a sampling system of the type described. Inthe preferred embodiment a rectifier stage is provided which is of atype which ensures that significant ripple will be present in therectified waveform. The average current value I_(a) is indicated on thevertical coordinate of the graph, the horizontal coordinate denotingtime. Current I_(c) is sampled at a rate f_(s), the sample points beingindicated as S1, S2, . . . Sn in the figure. For reasons to be morefully explained hereinafter it is important to the proper operation ofthe invention that sampling occur asynchronously with the alternatingcurrent Ic, that is, that the sampling frequency fs not be a harmonicfrequency of triple the frequency of the circuit current I_(c) in anyphase. "Harmonic" as used herein should be taken to mean harmonicfrequencies which are both above and below 3f.

It will be understood that individual samples do not for the most partproduce a signal truly indicative of average current value I_(a).Moreover, due to the error inherent in any system and particularly dueto the fact that only discrete, predetermined current value signals canbe outputted by conversion register 25, it will be appreciated that asingle sampling of the current such as occurs at time S1 will produce asignal which is only an approximation of actual sampled instantaneous,average, RMS, or other current value which is sought. However, by takinga large number of samples at irregular points on the waveforn the errorswhich occur due to the discrete signal levels in the signal processingsystem are effectively nulled out, and the accuracy of the systemgreatly enhanced.

The alternating current I_(c) flowing in conductors 10 induces similarsignals in current transformers CT1, CT2 and CT3 which are applied torectifier stage 16. The rectifier stage outputs a signal whichconstitutes an envelope of the half-wave rectified 3 phase currents inconductors 10. Here it should be pointed out the time constant of theramp-like comparison signal 20 is far less than that of the frequency ofthe line current being sampled. Referring to waveform I_(c) of FIG. 3,sampling periods S1-S4 are shown superimposed thereupon. For clarity ofexplanation these are shown in idealized form and at a lower frequencythan actually required, and it will be recognized that the duration ofthe samples themselves is small with respect to the period of thewaveform I_(c) so that the sampling may be considered almostinstantaneous in nature.

The current sensed during a sample period is represented by a digitalsignal which is a function of current value. By using a nonlinearreference waveform 20 such as an exponential curve a nonlinearrelationship is established between the level of sensed current and thedigital signals which represent it, and in this manner a constantresolution of the sensed current is maintained. Due to the steep slopeof the sampling waveform during the initial part of the sampling perioda rather small differential in time, or equivalently a small outputpulse width, corresponds to a relatively large sampled current.Conversely, lower-valued current samples give rise to longer outputsignals, whose durations are determined by the latter portion ofwaveform 20 which has a much lesser slope and therefore small deviationsin current value result in disproportionately larger changes in theduration of the resulting COMP pulse. The pulse-generating system istherefore considerably more sensitive to current levels for lower valuesof sampled current than for higher values.

FIG. 3 illustrates how the durations of the pulses outputted bycomparator 18 are inversely related to the level of current I_(c). Thedurations of the pulses in the Figure is exaggerated for purposes ofillustration. It is anticipated that the time duration of the pulsesoutputted by comparator 18, and indicated at the COMP line of FIG. 3,will be no greater than the duration of the sample time and in mostinstances somewhat less although when current envelope value remainshigh the comparator output may be continuously held in its "low" state.The first COMP pulse is somewhat shorter than the second, correspondingto the greater magnitude of the composite waveform during sample S1. Forsample S3, however, current I_(c) has increased substantially; hence theCOMP pulse is substantially shorter in duration. At sample S4, currentI_(c) has diminished from the value of sample S3 but is somewhat greaterthan that of samples S1 or S2. Accordingly, the fourth COMP pulse has aduration less than the first and second pulses. The COUNTER pulses shownon the third line of FIG. 3 occur at substantially the same time as theCOMP pulses. It should also be noted that although depicted as serial,COUNTER pulses occur simultaneously on four output lines. The longer theCOMP pulse, the higher the numerical value which the COUNTER pulsesrepresent. With the present system, higher numerical values correspondto lower current values, and conversely.

With an economical four-bit counter or microcomputer, however, themaximum number of values which can be represented is 16. Conversionregister 25, which may for instance be a read only memory (ROM) or thelike, receives the binary COUNTER signal and responds by outputting avalue which is assigned to the received four-bit signal. In a preferredembodiment utilizing a programmable microprocessor, conversion register25 is comprised by a simple look-up table programmed with appropriatevalues such as these in Table 1.

Returning to FIG. 1, counter 27 flips switch 26 so that every nth samplepulse is directed to the lower or "overcurrent" signal path, includingarithmetic unit 28. The value of n depends upon which TRIP CLASS inputof counter 27 is enabled. For a higher TRIP CLASS n becomes larger,corresponding to a lower rate of sampling by the overcurrent path, andthe time required to trip the circuit increases. The TRIP CLASS inputsmay for instance correspond to NEMA classes 10, 20 and 30 which requirethe device to trip a 600% overcurrent in 10, 20 or 30 secondsrespectively. During the intervening n-1 pulses counter 27 causes switch26 to direct current sample signals to arithmetic unit 35.

A threshold value I_(t) is subtracted from the CONV REG signals producedby the conversion register 25 by arithmetic unit 28 so that the ultimatevalue outputted to accumulator 29 reflects the difference between eachsample value and the threshold.

                  TABLE I                                                         ______________________________________                                        Count       Assigned Value                                                    ______________________________________                                        0           255                                                               1           192                                                               2           145                                                               3           109                                                               4           82                                                                5           62                                                                6           47                                                                7           35                                                                8           27                                                                9           20                                                                10          15                                                                11          11                                                                12           9                                                                13           6                                                                14           5                                                                15           4                                                                ______________________________________                                    

The result, which may be either a positive or a negative quantity, isapplied to accumulator 29 whose contents are represented in idealizedform at the ACCUM line of FIG. 3. As the accumulator receives eachcurrent-related signal from conversion register 25 it stores the signal,adding it algebraically to signals produced during previous samplingperiods. In addition the fed-back "thermal memory" signal increases,adding to the current threshold value I_(t).

It will be recognized that lower levels of current I_(c) producerelatively small CONV REG signals and result in small, or negative,increments in the accumulated value ACCUM; while large values of currentproduce larger CONV REG signals resulting in increases in theaccumulated signal such as that which occurs at the time of sample S3.While the ACCUM signal level in accumulator 29 is represented in analogform, it will be recognized that the retention and adding of suchsignals can be accomplished in various ways. The accumulator could, forinstance, be a capacitor which stores charge from supplied signals; or,as in a preferred embodiment, a RAM memory which receives sequential,digital signals in which case the cumulative value of eight-bit wordsignals is represented by the vertical height of the ACCUM curve.

If the ACCUM signal exceeds a second, trip threshold value T_(t), a TRIPsignal is produced by comparator 37 which causes contacts 11 to open. Inpractice the original TRIP signal is small in magnitude and must beamplified by one or more amplifying stages. As soon as the TRIP signaleffects the opening of contactor 11 current flow ceases. The samplingactivity continues, however but since I_(i) is zero the quantity (I_(i)-I_(t)) is negative and accumulator 29 is decremented accordingly. TheTRIP signal is fed back to TRIP CLASS counter 27 to effect adecrementing of the accumulator at a desired rate, and is applied toTRIP SELECT unit 37 to reduce the trip threshold T_(t) to a lower value.

In a presently preferred embodiment the output signal applied to theelectromechanical circuit interrupting stage comprises a series of"keep-alive" or "watchdog" pulses. In the simplified schematic diagramof FIG. 1, pulses from CLK are constantly applied to output circuit 15,and serve to maintain the energization of winding 14. A "trip" signaloutputted by comparator 38 in effect inactivates circuit 15, so that theCLK pulses are no longer transmitted to the relay. Here it should benoted that the relay shown in idealized form by winding 14 andnormally-closed contacts 13 may comprise various combinations ofwindings and contacts, and control or latching circuits, as areappropriate for the application.

Due to the absence of the "watchdog" output pulses the system contactscannot reclosed and this condition persists until the accumulator isdecremented to a predetermined value. Depending upon how soon the systemis reenergized a residual count will exist within the accumulator sothat a correspondingly smaller overcurrent signal count is required totrip the system again. This reflects the fact that the protected loadhas not fully cooled down from its pre-trip operation and thereforecannot tolerate as large and/or prolonged an overcurrent condition aswould be the case if the load device were "cold," that is, at ambienttemperature.

FIG. 4 represents the composite circuit current envelope when anunbalanced phase condition is present. The overall waveform isdistorted, and substantial changes occur from one sampling period to thenext. Such substantial changes, occurring in a relatively short time,produce differences between sampled signals sufficient to increment theJ counter of FIG. 1 and ultimately cause the value of the currentthreshold to be lowered in order to compensate for the changed currentcharacteristics. In this manner the illustrated system can detectunbalanced phase conditions, including single phase operation, withoutthe need for separate detection or signal processing equipment andwithout using differential signal processing by making use of currentsignals which are already generated within the system.

FIG. 5 illustrates in further detail a presently-preferred embodiment ofthe invention in which many of the signal processing functions arecarried out by a microcontroller. In a successfully tested embodiment amodel COP402 microcontroller, manufactured by the National SemiconductorCorporation of Santa Clara, Calif. was utilized in conjunction with aseparate erasable programmable read-only memory although, of course, itis anticipated that as it is produced in large numbers it will utilize acustom read-only memory. The microcontroller, herein designated at 40,utilizes the external oscillator option recommended by the manufacturerincluding a resonant crystal XTAC and capacitors C1, C2. Resistors R1aand R1b are coupled across the crystal as shown. Resistors R2-R8 arecoupled to microcontroller inputs L₀ -L₆, respectively. A power supplycircuit 41 providing a source of unregulated 8 volts and regulated 5volts is coupled between the GND and +5 terminals of themicrocontroller. Switches S1, S2 and S3 are coupled between a point ofreference potential and programming terminals G1, G2 and G3 for settingthe status, or trip class, of the microcontroller. Switches S1-S3 andsimilar devices coupled to terminals L₀ -L₆ may comprise individualjumpers or other easily manually-operated mechanisms which areaccessible to a user of the apparatus for presetting the unit inaccordance with field applications of the control, for instance, toallow or to prevent the resetting of the system subsequent to a trip orto tolerate a certain degree of imbalance in order to allow the unit tobe used with single-phase current. One of the buffered outputs D₃ iscoupled to a light emitting diode LED1 for enabling the diode in thepresence of an overcurrent condition, when a trip is impending, therebyproviding a visible indication of the state of the current monitoringsystem. As customary, LED1 is connected to a source of potential, here 5volts, through a resistor R9. A second light emitting diode LED2 isprovided for indicating a tripped condition.

The driver circuit indicated at 15 of FIG. 1 is more fully disclosed inFIG. 5, the quiescent or "no trip" output being a pulse train 42 whichis AC coupled from terminal D2 by capacitor C3 and a resistor R10 foroperating a relay set winding 14a and reset winding 14b. Current for theset winding flows through transistor Q₁, whose base is isolated from"no-trip" pulse train 42 by diode CR₁. The base is coupled to a sourceof reference potential through capacitor C₄ and to a source of biaspotential through resistor R11. A clamp diode CR₂ is coupled in shuntabout winding 14a. A further set of contacts 13 (FIG. 1) are alsooperated by winding 14a. The normally-open contacts 13a and 13aa arecoupled so that contacts 13a shunt capacitor C₄, and contacts 13aa liebetween a light emitting diode LED2 and ground. The cathode of LED2 isfurther coupled to terminal L7 of the microprocessor by resistor R₁₂,providing a feedback path for indicating the state of the relay tomicroprocessor 40.

The reset winding 14b has another diode CR₃ coupled about it, and liesin series with second transistor Q₂. A gated device such as thyristorSCR₁ couples the emitter terminal of Q2 to a point of referencepotential while normally closed contacts 13b lie in series with aresistor R13.

In operation, when the system is monitoring normal current flow a trainof "watchdog" pulses 42 is constantly produced at terminal D2. The pulsetrain biases SCR₁ into conduction so that reset winding 14b isenergized, causing contacts 13b to be closed. Diode CR₁ conducts thenegative-going edges of pulse train 42, discharging capacitor C4 andmaintaining the base terminal of transistor Q₁ at a low voltage level.This ensures that Q₁ will not be biased on by the 5 volt supplyconnected through resistor R11. When a TRIP signal occurs, pulse train42 stops and the gate signal is removed from SCR₁. At the same timecapacitor C4 charges, enabling transistor Q1 and energizing winding 14a.This causes contacts 13 to open cutting off current flow through winding12 and opening contacts 11. At the same time contacts 13aa are closed tocomplete a circuit through LED2, which lights to indicate a "tripped"condition and a ground or zero voltage is applied to input terminal D₅,informing the microprocessor that a "tripped" condition exists.

The values for resistor R10 and capacitor C3 are selected to form an RCcircuit whose time constant is appropriate for maintaining an adequategating voltage on the gate terminal of SCR₁, and for maintainingcapacitor C4 in a relatively discharged state. By tuning the RC circuitto the periodicity of the pulse train, control circuit 15 can be maderesponsive to a pulse train of a particular frequency, and to improperfluctuations by the pulse train. This characteristic provides a safetyfeature in the event that microprocessor 40 malfunctions in such amanner as to output irregular pulses to the control circuit or if thevoltage level of the pulses drops to an unacceptably low level,indicating an abnormally low voltage level within the control whichcould, in turn, lead to signal processing errors.

In the foregoing manner it will be understood that TRIP signal outputtedby FIG. 5 is in actuality the absence of a pulse train havingpredetermined characteristics with regard to amplitude, frequency, andto some degree, pulse configuration. By requiring that the controloutput a particular form of signal in order to maintain thecircuit-protecting interrupter in its closed condition, the outputcircuit thus provides a fail safe function and enhances the integrity ofthe overall circuit protection system.

The power supply for systems of the present type is of particularinterest, as it presents a substantial design challenge due to the factthat devices such as the industrial overload relay ilustrated should beadaptable for use with AC power of from 100 to 500 volts and forfrequencies of both 50 and 60 Hz, while producing a constant low-voltageregulated output. At the same time, due to the relatively low price ofsuch devices the cost of the power supply must be low, ruling outcomplex regulation systems of conventional design. The present inventorhas resolved these problems by utilizing a phase-angle controlledthyristor arrangement, without the need for complex firing andcommutation circuits or elaborate gate-timing systems.

Referring again to FIG. 5, a pair of input terminals T₁ and T₂ areprovided, across which an AC voltage from 100 to 500 volts may beimpressed. An RC filter circuit comprising resistors R₂₅ and R₂₆ inseries, with capacitor C₇ coupled between the intersection of theresistor and terminal T₂. The series combination of resistors R₂₇, R₂₈and capacitor C₈ are also connected across the input terminals, withdiode CR₁₀ coupled in shunt about resistor R₂₈ and a Zener diode CR₁₁connected between the intersection of resistors R₂₇ and R₂₈ and terminalT₂. The intersection of capacitor C₈ and resistor R₂₈ is coupled to thegate terminal of thyristor SCR₂ while the anode of the thyristor iscoupled to terminal T1 through resistors R₂₅ and R₂₆.

A second Zener diode CR₁₂ extends between the cathode of SCR₂ andterminal T₂, in parallel with capacitor C₉. An ordinary low-voltagetransistorized voltage regulator IC₁, which may be a conventionalintegrated circuit regulator for transforming a regulated level ofapproximately 8 volts to a closely-regulated 5 volt output, is coupledto the output of the circuit as shown and a shunt regulator LC2connected across the power supply output in order to protect LC1 fromfed-back overvoltages which may arise on the 5 volt bus. Capacitor C10is placed in parallel with LC2 to provide a filtering action.

An example of an AC input voltage is shown at a line V_(in) of FIG. 6.The maximum value V_(m) of the waveform may vary considerably, and it isintended that the present power source be usable over a range of supplyvoltages of from 100 to 500 volts, and for both 50 and 60 Hz supplies,in order to accommodate the various supply voltages which are utilizedin diverse geographical areas and applications. Voltage V_(1n) isimpressed across the series combination of resistor R₂₅ and capacitorC₇, thereby applying a filtered AC waveform to the anode of SCR₂. At thesame time, the waveform is impressed across the series combination ofresistor R₂₇ and Zener diode CR₁₁.

The voltage across CR₁₁ is shown at line V_(z) of FIG. 6. It will beunderstood that the voltage scale for line V_(z) has been expandedconsiderably, owing to the fact that the threshold voltage of the diodeis considerably less than maximum voltage V_(m), and is commonly on theorder of approximately 22 volts. Owing to the well-known characteristicsof Zener diodes, the voltage thereacross remains substantially constantduring the positive-going half cycles of the supply voltage.

During positive-going half cycles of the supply voltage current flowsthrough resistor R₂₈ to the upper plate of capacitor C₈, charging thecapacitor. The capacitor voltage, illustrated at line V_(c8) of FIG. 6,exhibits a typical ramp-like characteristic, whose time constant isdependent upon the values of the R-C circuit formed by resistor R₂₈ andcapacitor C₈. As charging current flows through resistor R₂₈ voltageacross capacitor C₈, and therefore the voltage on the gate of SCR₂,builds up gradually in a ramp- or exponential-like fashion.

In order to gate SCR₂ on, the voltage at the gate terminal thereof mustbe sufficiently greater than the voltage at the cathode to allow thenecessary gating current to flow. Therefore, although the gatingoperation is actually based upon current, for purposes of the presentexplanation voltage levels will be referred to.

The gating voltage necessary to cause SCR₂ to fire is then a function ofthe voltage across capacitor C₉, which serves as filter and currentsource for the power supply. When the circuit commences operation thevoltage across capacitor C₉ is relatively small, and a relatively lowgating voltage is required to cause SCR₂ to conduct. This relatively lowvoltage is achieved early in the cycle, before the ramp-like voltageupon capacitor C₈ has built up to a great degree. This relatively earlyfiring point is designated at φ₁ at line V_(c8). Once gated on, the SCRcontinues to conduct for the balance of the positive-going waveform,that is, up to 180°. The on-time of the SCR is then represented by theperiod Δ₁. As the source voltage V_(in) reverses direction and thevoltage on the cathode of Zener diode CR₁₁ goes to zero diode CR10becomes foreward biased and discharges capacitor C8, terminating theramp-like gating waveform and allowing SCR₂ to be commutated by thevoltage reversal.

Current which flows during the time interval Δ₁ charges capacitor C₉,thereby providing a voltage level to the 8 v bus and to transistorizedvoltage regulator VREG, which in turn maintains a consistent, 5 voltregulated output. The Zener diode CR₁₂ keeps the voltage on capacitor C₉from exceeding the desired limit, particularly under startup conditionswhen the lack of initial voltage on capacitor C₉ may cause a voltagesurge to occur.

As the voltage upon the capacitor builds up toward the desired 8 volts,a longer time is required before capacitor C₈ is charged to thenecessary gating voltage. As shown by the second waveform on line V_(C8)the required, higher gating voltage is not attained until time φ₂, withthe result that commensurately shorter conduction time Δ₂ is providedduring which charging current can flow through SCR₂ to capacitor C₉.This feedback effect automatically self-limits the charging of capacitorC₉, so that a relatively constant voltage is maintained.

It will now be appreciated that, depending upon the load and thereforethe rate at which current is drained from capacitor C₉ the firing pointφ of SCR₂ will automatically vary, resulting in a greater or lesserconduction time for SCR₂ and therefore a greater or lesser charging timefor capacitor C₉. In this manner a predetermined voltage level isautomatically maintained upon capacitor C₉ without the necessity forcomplex gating or control apparatus.

In order to obtain a precisely-regulated 5 volt power supply for thecontrol system, a 2-element final voltage regulator is used. A seriesregulator, designated IC₁, regulates the voltage supplied by capacitorC₉ to the 5 volt bus. In a presently preferred embodiment IC₁ comprisesa standard 3-terminal positive voltage series regulator identified inthe industry as no. 7805. Type 7805 regulators are available from manysources, any of which is deemed to be adequate for present purposes. Asis commonly known, while such devices produce a well regulated 5 voltoutput they require at least a moderately well regulated input on theorder of 8 volts. In practice, it has been found that the 8 volt outputarising across capacitor C₉ is maintained in the range of approximately7 to 11 volts over an input variation of 100 to 500 volts at terminalsT₁ -T₂. In addition, a second, shunt regulator IC₂ is provided in orderto accommodate high voltage surges which may be applied to the 5 voltbus from current transformers CT1-CT3. Such a shunt regulator is simplyrequired to conduct current from the 5 volt bus to ground, or reference,potential in order to keep the bus voltage from exceeding apredetermined limit, advantageously in the range of 5.25 volts. Whilevarious transistor regulators may be used a simple breakdown device,such as a precision Zener diode, may be utilized with the principlerequirement being the prevention of an inordinately high voltage surgeupon the 5 volt bus.

While the accuracy of regulation will be determined in part by thespecific component values, in a successfully tested embodiment, Zenerdiode CR₁₁ exhibited a breakdown voltage in the range of 20 volts, whileZener diode CR₁₂ had a breakdown voltage of about 13 volts.

It will now be recognized that the present invention comprises aneconomical yet accurate voltage regulator which is adaptable for useover a broad range of input voltages, and which exhibits a self-limitingfunction without the need for special sensing or timing circuits. Whilethe system is illustrated for use in combination with an industrialrelay, it is anticipated that it will find application in many othersystems in which reliable voltage regulation is required at low cost.

It will be readily appreciated by those skilled in the art that many ofthe elements shown in discrete form in FIG. 5 could easily be providedby one or more integrated circuits, and for those applications in whichthe described system is produced in quantity it is anticiated that manyamplifiers, transistors and resistors can be incorporated in a singlecustom integrated circuit. Such embodiments of the system are deemed tobe well within the skill and discretion of circuit designers.

As will be evident from the foregoing description, certain aspects ofthe invention are not limited to the particular details of the examplesillustrated, and it is therefore contemplated that other modificationsor applications will occur to those skilled in the art. It isaccordingly intended that the appended claims shall cover all suchmodifications and applications as do not depart from the true spirit andscope of the invention.

I claim:
 1. In an overload relay including means for monitoring currentflow in a circuit, electromagnetic interrupter means having contacts forbreaking said circuit and control means coupled to sensing means forcausing said interrupter to interrupt said circuit upon the detection ofan overcurrent condition, the improvement comprising:means forgenerating a train of substantially identical pulses; receiving meanscoupled to said electromagnetic interrupter and responsive to said trainof pulses for maintaining said interrupter in a closed position; meansfor terminating said train of pulses upon the detection of anovercurrent condition; R-C filter means connected between said controland sdid electromagnetic interrupter and responsive to a pulse train ofa particular frequency and magnitude for maintaining energization ofsaid electromagnetic means, said electromagnetic interrupter including:a first winding for opening said contacts, current control means inseries with said winding for controlling current flow therethrough, andmeans coupling said current control means to said R-C circuit; a secondcontrol winding for causing said contacts to close; second currentcontrol means coupled in series with said winding to control currentflow therethrough; and means coupling said second current control meansto said R-C circuit for causing current to flow through said secondwinding upon interruption of said pulse train.
 2. The invention definedin claim 1, wherein said first current control means is a thyristor, andwherein said second current control means comprises a transistor havingits emitter-to-collector circuit in series with said second winding, andhaving a resistor-capacitor network coupled to the base thereof; andunidirectional directing means coupling the base terminal thereof tosaid R-C circuit, whereby pulses of said pulse train serve to repeatedlydischarge the capacitor of said R-C circuit.
 3. In an overload relayincluding means for monitoring current flow in a circuit,electromagnetic interrupter means having contacts for breaking saidcircuit and control means coupled to sensing means for causing saidinterrupter to interrupt said circuit upon the detection of anovercurrent condition, the improvement comprising:R-C filter meansconnected between said control and said electromagnetic interrupter andresponsive to a pulse train of a particular frequency and magnitude formaintaining energization of said electromagnetic means, saidelectromagnetic interrupter including a first winding for opening saidcontacts, current control means in series with said winding forcontrolling current flow therethrough, and means coupling said currentcontrol means to said R-C circuit; a second control winding for causingsaid contacts to close; second current control means coupled in serieswith said winding to control current flow therethrough; and meanscoupling said current control means to said R-C circuit for causingcurrent to flow through said second winding upon interruption of saidpulse train.
 4. The invention defined in claim 3, wherein said firstcurrent control means is a thyristor, and wherein said second currentcontrol means comprises a transistor having its emitter-to-collectorcircuit in series with said second winding, and having aresistor-capacitor network coupled to the base thereof; andunidirectional directing means coupling the base terminal thereof tosaid R-C circuit, whereby pulses of said pulse train serve to repeatedlydischarge the capacitor of said R-C circuit.
 5. In an overload relayincluding a current sensing stage, a control stage coupled to saidcurrent sensing means, an interrupter stage including a pair of contactsfor interrupting a circuit, and a winding for operating said contacts,the improvement comprising:R-C filter means connected between saidcontrol stage and said interrupter stage and responsive to a pulse trainof a particular frequency and magnitude for maintaining energization ofsaid interrupter stage; solid-state control means connected in serieswith said winding for controlling current flow therethrough; meanscoupling said current control means to said R-C circuit; a secondcontrol winding for causing said contacts to close; second currentcontrol means coupled in series with said winding to control currentflow therethrough; and means coupling said second current control meansto said R-C circuit for causing current to flow through said secondwinding upon interruption of said pulse train.